The current tests for inactivated poliovirus vaccine (IPV) potency need standardization between laboratories. A collaborative study has been initiated to assess the suitability of candidate reference materials. Another aspect of the study is to assess the suitability of a candidate standard method for D antigen assay by ELISA, and to assess the proficiency of methods for potency estimation of IPV used by individual laboratories. The materials included in this study are two commercially prepared candidate reference materials; the primary reference PU78-02 currently used by several laboratories, vaccines from different manufacturers shown to be immunogenic in humans; IPV produced from Sabin strains and vaccines of poor in vivo potency. The study will be divided into two phases. The first phase which is in progress will involve in vitro assays and the second phase in vivo assays. An ELISA developed at CBER using as detection antibodies monoclonal antibodies specific for D antigen is the candidate standard method. We filled and lyophilized our in-house reagents for distribution to collaborators. Several tests were performed to validate the lyophilized reagents prior to mailing. We prepared a detailed SOP which included the dilution schemes for both practice antigens and test antigens. The collaborators have received an ELISA kit containing plates, polyclonal and monoclonal antibodies and monotypic antigen of all three poliovirus types for practice together with the SOPs and the results of our tests using the monotypic antigens. The test antigens labeled A-F were mailed at a later date. Testing is in progress. We are currently evaluating the Balb/c mouse as a model of in vivo potency as a separate study. We plan to propose using Balb/c mice as a candidate standard in vivo potency method for the second phase of the collaborative study. We will be involved in the analysis and reporting of the study at each phase.